In the post-genome era, proteomics has emerged as the next great scientific challenge. While methodologies vary widely, a near-universal first step for proteomic analyses of physiological samples (e.g., blood, serum, tissue extract, etc.) is removal of the non-relevant solution constituents (e.g., nucleic acids, lipids, etc.).1-4 A common method used to accomplish this task is protein precipitation. In this technique, one or more precipitants (organic solvents, salts, or pH modulators) is mixed with the protein-containing sample, which causes proteins to precipitate and settle to the bottom of the reaction vessel.1-3,5-8 After centrifuging, removal of supernatant, and washing in appropriate rinse solvents, the precipitate can be re-dissolved, and the now-purified solution can be used for subsequent processing and analysis.
The present invention discloses the development of an automated, microfluidic method for extracting proteins from heterogeneous fluids by precipitation. Although there have been myriad applications of microfluidic technologies to proteomics,9 to the inventor's knowledge, there have been no disclosures describing protein extraction by precipitation in microchannels. It is noted that precipitation has been used in channels to remove proteins for analysis of other analytes,10 but not as a technique to extract and collect proteins for further analysis. The inventors speculate that this deficit is a function of complexity and heterogeneity since in protein extraction by precipitation, liquids (samples, rinse solutions, etc.), solids (precipitates), and vapor phases (air for drying precipitates) all play a prominent roles and must be precisely controlled. These requirements seem like a poor match for the conventional format for microfluidics, enclosed microchannels. Thus, in the current work, we chose to use the alternative format of digital microfluidics (DMF).
In DMF, discrete droplets of sample and reagents are controlled (i.e., moved, merged, mixed, and dispensed from reservoirs) by applying a series of electrical potentials to an array of electrodes.11,12 Because of its reconfigurability and scalability, DMF has recently become popular for a wide range biochemical applications including cell-based assays,13 enzyme assays,14-16 protein profiling,17-19 and the polymerase chain reaction.20 More importantly, the DMF format seems well suited for complex procedures such as protein extraction by precipitation, as DMF can be used to precisely control liquid-, solid-, and gas-phase reagents in heterogeneous systems.21-24 
It would therefore be very advantageous to provide an automated method for extracting and purifying proteins from heterogeneous mixtures using digital microfluidics as this would facilitate high throughput extraction and screening of proteins.